1. Field of the Invention
The invention generally relates to a communications system. More particularly, the invention relates to a wireless communications device and a method of receiving radio frequency signals within a communications systems.
2. Description of the Related Art
One example of a communications system is a wireless communications system which can be a cellular mobile communications system. The cellular mobile communications system is implemented in a geographical area and logically divided into individual service cells. A fixed transceiver station such as a base station defines at least one cell and is connected to a base station controller. Mobile stations, such as hand-held or car-based cellular phones, move freely within the geographical area covered by a cell. The mobile stations not only move within a single cell, but also from one cell to a neighboring cell.
The base station handles all telephone traffic to and from those cellular phones which are currently located in the cell. The base station that serves a cellular phone is typically the one which is closest to the cellular phones and, thus, provides in many cases the best radio communications path to the cellular phones.
The cellular phones and the serving base station exchange radio signals in accordance with a communications protocol defined for a given communications system. The radio signals have frequencies within frequency bands that are assigned to the cells. In one example of a communications protocol, the radio signals can be structured in frames and channels.
In conventional Code Division Multiple Access (CDMA) systems, a pilot channel is defined for communications between the base stations and the cellular phones. The pilot channel carries no information, but provides the cellular phone, for example, with a reference for time, phase and signal strength. The cellular phone constantly evaluates the strengths of the pilot channels of the serving and neighboring base stations to determine potential base stations. When the strength of the pilot channel of the serving base station falls below a predetermined threshold and the strength of the pilot channel of the neighboring base station exceeds a predetermined threshold, a handoff procedure is initiated. The procedure that transfers the mobile station from one cell to another cell, without dropping a call or losing information, is often called xe2x80x9cSoft Handoff.xe2x80x9d
In many conventional Soft Handoff procedures, the base stations of neighboring cells use the same frequencies. This requirement, however, limits the number of mobile stations that can be served by one base station. For example, if two neighboring base stations operate at different frequencies, a so-called xe2x80x9cHard Handoffxe2x80x9d procedure typically takes place which causes a break in an existing connection and may result in a loss of information.
An aspect of the invention involves a wireless communications device for a communications system. The wireless communications includes an antenna which receives a first signal at a first radio frequency and a second signal at a second radio frequency and convert the first and second signals into a composite radio frequency (RF) signal. A first oscillator is operable to output a first oscillator signal at a first frequency, and a second oscillator is operable to output a second oscillator signal at a second frequency. A demodulator is coupled to receive the composite RF signal and the first and second oscillator signals. The oscillator signals are selected so that the demodulator generates a low frequency signal with components of the first and second signals occupying a common frequency band.
Another aspect of the invention involves a wireless communications device having a first input configured to receive an input signal which comprises a first component allocated within a first frequency band and a second component allocated within a second frequency band. A first oscillator is configured to generate a first oscillator signal at a first oscillator frequency, and a second oscillator is configured to generate a second oscillator signal at a second oscillator frequency. A mixer is configured to receive the input signal, the first oscillator signal and the second oscillator signal, and to convert at least a portion of the first component and at least a portion of the second component to a third frequency band.
A further aspect of the invention involves a device having at least a first terminal which is configured to receive a first signal within a first frequency band from a first source and a second signal within a second frequency band from a second source. At least a second terminal is configured to receive at least a first reference signal and a second reference signal. A modulator in communication with the first and second terminals is configured to generate a first difference component within a third frequency band. The first difference component comprises the difference between a portion of the first signal within the first frequency band and the first reference signal. The modulator is further configured to generate a second difference component within the third frequency band, the second difference component comprising the difference between a portion of the second signal within the second frequency band and the second reference signal.
Another aspect of the invention involves a wireless communications device having a first input to receive an input signal which comprises a first component having a first frequency allocated within a first frequency band and a second component having a second frequency allocated within a second frequency band. A first oscillator is configured to generate a first oscillator signal at a first oscillator frequency, and a second oscillator is configured to generate a second oscillator signal at a second oscillator frequency. A mixer is configured to receive the input signal, the first oscillator signal and the second oscillator signal, and to convert at least a portion of the first component and at least a portion of the second component into a third frequency band. The portion of the first component has a first difference frequency corresponding to a difference between the first frequency and the first oscillator frequency, and the portion of the second component has a second difference frequency corresponding to a difference between the second frequency and the second oscillator frequency. The first difference frequency is approximately equal to the second difference frequency, both located within the third frequency band.
A further aspect of the invention involves a method of receiving radio frequency (RF) signals with a wireless communications device that is operable in a communications system. The device receives a first signal within a first frequency band from a first source, and a second signal within a second frequency band from a second source. Further, the device transforms the first and second signal into a third frequency band, and processes the frequency-transformed first and second signals in order to maintain communications with the first and second sources.
Another aspect of the invention involves a method of receiving radio frequency (RF) signals. A first RF signal has a first radio frequency and originates from a first transmitter station, and a second RF signal has a second radio frequency and originates from a second transmitter station. The first and second RF signals are received and converted into a composite signal. A first oscillator signal is generated having a first oscillator frequency which is selected to have a first frequency difference to the first radio frequency. A second oscillator signal is generated having a second oscillator frequency which is selected to have a second frequency difference to the first radio frequency. The composite signal is mixed with the first and second oscillator signals to generate an intermediate frequency signal. The intermediate frequency signal comprises a component of the first RF signal and a component of the second RF signal with the components being located within a common frequency band. The intermediate frequency signal is processed to generate a first baseband signal and a second baseband signal. The first baseband signal corresponds to the first RF signal and the second baseband signal corresponds to the second RF signal.
For purposes of summarizing the invention, certain aspects, advantages and novel features of the invention have been described herein. Of course, it is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.